Bicycle rear suspension

ABSTRACT

A bicycle comprises a frame ( 10 ) a rear wheel ( 14 ) and a rear suspension, which comprises a swing arm ( 17 ) carrying the wheel and two pivot links ( 18, 19 ) spaced from one another in the fore and aft direction of the bicycle and coupling the arm to the frame to form, together with the frame, a four-bar linkage movable between a first end settings in an unloaded state of the suspension and a second end setting in a loaded state of the suspension. The links ( 18, 19 ) are each pivotably connected with the frame outwardly of the wheel circumference and are arranged so that on movement of the linkage from the first to the second end setting an instantaneous pivot center ( 21 ) defined by the intersection of two axes ( 22, 23 ) each containing the pivot points of a respective one of the links ( 18, 19 ) moves downwardly and rearwardly with respect to the fore and aft direction.

The present invention relates to a bicycle and has particular referenceto a rear wheel suspension in a bicycle.

Bicycles intended for off-road and other rough terrain uses commonlyhave rear suspension systems permitting sprung movement of the rearwheel relative to the bicycle frame. A basic system is represented by arearwardly extending swing arm carrying the rear wheel and pivotablyconnected to a seat tube or down tube of the bicycle frame. A spring anddamper unit is coupled between the spring arm and the frame to controlpivot movement of the arm under suspension travel. The swing arm isusually a sturdy cast or fabricated member which is suitably stiff inbending and which can incorporate robustly formed journals for pivotconnection to the frame and the spring and damper unit. The sturdinessof the swing arm is highly desirable for acceptance of the shock loadsacting on the suspension during off-road use, but the simple arcexecuted by the swing arm during suspension travel imposes a compromisewith respect to the suspension behaviour. In particular, the drive forcetransmitted to the rear wheel by the usual chain-and-sprockettransmission during a power stroke applied by a rider tends to pivot upthe swing arm and wheel independently of bump response of thesuspension. The arm is then able to pivot down again in the intervaluntil the next power stroke, which results in a bobbing effect or atleast some degree of feedback from the foot pedal cranks which usuallyfeed the drive force to the transmission.

These disadvantageous effects can be counteracted by more complexmulti-element suspension linkages, of which the four-bar linkage isparticularly effective from the viewpoint of controlling rear wheelmovement in the course of suspension travel. Such a linkage can,depending on the position of its pivot centre, generate acounterbalancing force tending to pull the rear wheel down in oppositionto the tendency of the wheel to move up during the power stroke of therider. Generation of the counterbalancing force requires a relativelyhigh pivot centre of the linkage and such a high location also confersthe advantage that the rear wheel path has a more rearward initialdirection to enable a better response to bumps. However, a high pivotcentre location introduces the problem of disturbance to the pedalaction by a constantly changing chain length, thus pedal feedback, whenthe suspension has to cope with a continuously bumpy surface. Inaddition, large changes in chain length, i.e. changes in effectivelength caused by increase and decrease in the length of an idle zone ofthe chain, cannot be readily absorbed by conventional gear changingsystems based on chain displacement between coaxial sprockets ofdifferent diameter.

Problems of this nature can be resolved by designing the four-barlinkage so that its instantaneous pivot centre moves down and backtowards the pedal crank axis as the linkage displaces under progressive,bump-induced suspension travel. One such linkage is disclosed in U.S.Pat. No. 5,509,679 and subsequent continuation specifications, in whichthe linkage is composed of two rearwardly extending lower arms disposedone on each side of the wheel and pivotably connected to a frame seattube, two upwardly extending parallel upper arms similarly disposed oneon each side of the wheel and pivotably connected to the lower arms nearthe wheel axis and a short upper link pivotably coupling the upper armsto the seat tube at a spacing above the point of connection of the lowerarms. The length of seat tube between the two pivot connectionsrepresents the fourth element of the linkage. A spring and damper unitis angled between the short upper link and the frame. The relationshipand dimensions of the constituent elements of this four-bar linkage havethe result that the instantaneous pivot centre—represented by theinstantaneous point of intersection of the two notional axesrespectively containing the fulcra of the short upper link and thefulcra of the lower arms—does indeed displace rearwardly and downwardlyduring linkage compression. However, the length of the lower arms andthe point of pivot connection to the frame impose a limit on the extentof vertical shift of the instantaneous pivot centre. More significantly,the provision of paired upper and lower arms, the lengths of these armsand the pivot interconnection thereof near the rear wheel axis detractfrom the rigidity of this part of the linkage and the suspension as awhole tends to be appreciably less robust and less resistant to lateralflexing than suspension systems based on a single swing arm. The sitingof the spring and damper unit also imposes constraints and, in practice,modification of the frame to a more complex shape has been needed toaccommodate a spring and damper unit offering an effective stroke.

Other permutations of four-bar linkage rear suspensions in commerciallyavailable bicycles include a design with a triangular wheel carrierframe representing one element of the linkage and articulated to thebicycle frame, part of which forms another element of the linkage, by ashort upper link and a short lower link below the upper link. The upperlink has an extension providing a coupling point for a spring and damperunit connected to the bicycle frame down tube. This design yieldsprimarily a downward displacement of the instantaneous pivot centreduring suspension bump response, but without significant approach to thepedal crank axis, and the triangular carrier frame is a comparativelybulky component of the linkage. Moreover, lateral stiffness isprejudiced by the vertical relationship of the two links, whichco-operate to define a hinge zone. A further known design overcomes thebulkiness of the carrier frame by provision of a robust swing arm whichis articulated to the bicycle frame by a short upper link directedupwardly from the swing arm and a short lower link arranged below theupper link and directed forwardly and downwardly from the arm. Althoughthe swing arm itself possesses structural rigidity, resistance tolateral flexing is again compromised by the disposition of the two linksapproximately in vertical alignment and the instantaneous pivot centreof the linkage moves forwardly rather than rearwardly during suspensioncompression, thus producing displacement of the pivot centre in adirection actually opposite to that desired to counteract bobbing.

The invention therefore has as its principal objective the provision ofa bicycle with a rear suspension system which achieves the desireddegree of suspension compliance in conjunction with resistance tobobbing, but without a penalty in terms of structural rigidity,especially resistance to lateral flexing.

A supplementary object is the provision of a rear suspension linkagewhich can be readily accommodated in a conventional bicycle framedesign, thus without obliging special shaping, and for which a degree offreedom exists in specific dimensioning and disposition of the linkageelements. A further supplementary object is the design of a suspensionwith commonality of some parts, so as to ease production costs. Yetanother supplementary object is the creation of a suspension layout withscope for variable mounting of springing and damping means to enablevariation of springing and damping rates by simple measures.

Other objects and advantages of the invention will be apparent from thefollowing description.

According to the present invention there is provided a bicyclecomprising a frame, a rear wheel and a rear suspension which comprises aswing arm carrying the rear wheel and two pivot links spaced from oneanother in the fore and aft direction of the bicycle and coupling thearm to the frame to form therewith a four-bar linkage movable between afirst end setting in an unloaded state of the suspension and a secondend setting in a loaded state of the suspension, wherein the links areeach pivotably connected with the arm and the frame outwardly of thewheel circumference and are arranged so that on movement of the linkagefrom the first to the second end setting an instantaneous pivot centredefined by the intersection of two axes each containing the pivot pointsof a respective one of the links moves downwardly and rearwardly withrespect to said direction at a first rate over an initial part of thetravel of the pivot centre and at a second rate reduced in relation tothe first rate over a subsequent part of the travel of the pivot centre.

Through use of a swing arm and disposition of the pivot connections ofthe linkage outwardly of the wheel circumference the suspension can bebased on a wheel carrier member meeting all requirements of strength,rigidity and relative ease of manufacture. The swing arm can be a cast,forged or fabricated component of desired shape, for example cranked orbent in its length to provide clearance for other components, such as amain sprocket wheel of a chain-and-sprocket drive of the bicycle. Thetwo links can be kept short so that they can be constructed to beparticularly stiff in bending and thus maintain overall rigidity of thefour-bar linkage consisting of the rigid swing arm, the normallyinherently stiff frame and the links. The rigidity of the linkage isenhanced by the spacing of the links in the fore and aft direction ofthe bicycle so that the linkage does not contain a hinge zone formed bya vertical or more-or-less vertical plane containing the pivot points oftwo of the links. At the same time, however, these links are disposed sothat on upward movement of the rear wheel under bump response of thesuspension the instantaneous pivot centre of the linkage movesdownwardly and rearwardly with respect to the fore and aft direction.This provides the desired pivot behaviour of the four-bar linkage withrespect to control of the direction of wheel movement to counteract thebobbing effect occurring, as described in the introduction, bytensioning of the chain of the chain-and-sprocket drive. In addition,because the movement of the instantaneous pivot centre over an initialpart of its travel takes place at a first rate and over a subsequentpart of its travel at a second rate reduced in relation to the firstrate it is possible for the pivot centre to initially move relativelyquickly when upward pivotation of the suspension occurs and thereafterto move at a slower rate.

The link orientations can be selected so that a rearward one of thelinks pivots through a greater angle than a forward one of the linksduring an initial phase of movement of the linkage from the first to thesecond end setting and through generally the same angle as the forwardlink during a final phase of that movement. Consequently, the initialmovement of the pivot centre can be substantially along the axis of theforward link with only slight change in the position of that axis. Forpreference, the locus of the instantaneous pivot centre during itsmovement describes a first arc which is substantially convex upwardlyand a second arc which is substantially concave upwardly. This locusessentially results from an initial upward and subsequent downwardpivotation of the forward link. During the movement of the pivot centre,the locus thereof can be such as to pass through the pivot point of theforward one of the two links at the connection thereof with the frame,in particular at the point of transition from the upward to downwardmovement of that link or from the convex to the concave arc. Thedirection of movement of the pivot centre can be generally towards theaxis of a pedal crank drive of the bicycle and the centre is preferablydisposed adjacent to the axis of that drive in the second end setting ofthe linkage. This has the effect that the path of the rear wheel axisduring compression of the suspension initially gives rapid increase inthe distance between the axes of the rear wheel and the pedal crankdrive, thus growth in length of a chain coupling a drive sprocket anddriven sprocket respectively associated with the pedal crank drive andthe rear wheel, and subsequently virtually no growth.

The orientations of the two links are preferably selected to be suchthat the rearward link extends forwardly and the forward link rearwardlyfrom the arm with respect to the fore and aft direction of the bicycle,the forward link preferably also extending downwardly from the arm. Theangular disposition of the links when they extend forwardly andrearwardly in the manner described is for preference such that theyextend convergently in direction away from the rear wheel in the firstend setting of the linkage. A particularly compact disposition of thelinks satisfying the requirements for their relative positioning isachieved if the rearward link is connected to the frame at a seat tubethereof and the forward link to the frame at a down tube thereof.

With respect to construction, the links can be substantially identicalin shape and size so as to achieve a significant cost advantage inproduction as well as savings in procurement and stock-holding. Theconnection of the links to the swing arm and the frame can be effectedby connecting means comprising, for example, deep-groove full complementbearings which provide substantially play-free pivot location of thelinks. The links can be machined or moulded components so that, forexample, bearing fit can be accurately controlled without need forsubsequent welding or heat treatment. Similarly, the frame and swing armneed only be drilled to accommodate pivot pins for the bearings. Forpreference a single continuous pivot pin is provided at each pivotpoint, which further contributes to the lateral stiffness of thesuspension particularly at the locations most susceptible to play, i.e.the coupling points of the elements of the four-bar linkage. Other formsof connecting means are possible, including location of pins in thelinks and bearings in the arms and frame.

The suspension can be completed by a spring and damper unit pivotablyconnected with the arm and frame to provide sprung and damped travel ofthe linkage between its end settings. Location between the arm and frameallows scope for an effective stroke of the unit without obligingspecial shaping of members of the frame to the disadvantage ofstructural simplicity and strength and/or aesthetics. In addition toeasier accommodation of the unit, provision can be made for adjustmentof the position of the unit relative to the arm or frame for variationin the springing and damping rates. Due to the feature of rearwardmovement of the instantaneous pivot centre of the four-bar linkage undermovement of the suspension, the initially large spacing of the wheelaxis from that centre, which yields a large wheel to spring/damperratio, subsequently reduces to correspondingly reduce the ratio.Consequently, by adjustment of the inclination of the line of action ofthe spring and damper unit relative to the axis of the forward one ofthe links, which changes the perpendicular distance of that line ofaction from the pivot centre, the wheel to spring/damper ratio can bealtered in accordance with specific requirements. The adjustmentfacility can be such as to be readily carried out by the rider.

An embodiment of the present invention will now be more particularlydescribed by way of example with reference to the accompanying drawings,in which:

FIG. 1 is a schematic elevation of the rear part of a bicycle embodyingthe invention; and

FIGS. 2 a–e are schematic diagrams showing five progressive stages ofmovement of a rear suspension linkage in the bicycle part illustrated inFIG. 1.

Referring now to the drawings there is shown part of a bicycle, forexample a bicycle intended for off-road use (“mountain” bicycle),comprising a frame 10 with a crossbar 11, down tube 12 and seat tube 13.The crossbar 11 and down tube 12 may be united into a single componentto the right of FIG. 1 and, whether so united or remaining separate, areconnected to a steering assembly and front wheel forks or suspension(not shown) of the bicycle. At the junction of the down tube 12 and seattube 13 there is provided a bearing location for a pedal crank drivefixedly connected with a drive sprocket of a chain-and-sprocket drivetransmission (not shown) to a rear wheel 14 of the bicycle. A drivensprocket of the transmission is fixedly connected with an axle of therear wheel and drive is transmitted from the drive sprocket to thedriven sprocket by way of an endless chain. In the case of a drivetransmission provided with gearing, several coaxial drive or drivensprockets of respectively different diameter may be provided togetherwith a chain displacing device (derailleur mechanism) for displacing thechain between the sprockets of different diameter. Pedal crank drivesand chain-and-sprocket drive transmissions are conventional componentsof bicycles and accordingly are not illustrated in FIG. 1. However, theaxis of the pedal crank drive and drive sprocket(s) is denoted by 15 andthe axis of the rear wheel axle and the drive sprocket(s) by 16.

The rear wheel 14 is suspended by a rear suspension comprising a swingarm 17 which carries an axle rotatably mounting the rear wheel, anupper, rearward link 18 extending forwardly of the arm and pivotablyconnecting the swing arm with the seat tube 13 and a lower, forward link19 extending rearwardly of the arm and pivotably connecting the swingarm with the down tube 12. The swing arm, links and the part of theframe between the pivot connection points of the links represent theelements of a four-bar linkage. The swing arm 17 is fabricated fromsheet aluminium or aluminium alloy, but could equally well be producedfrom other materials and could be cast, forged, machined or constructedin any other suitable manner. It is cranked in its length to rise abovethe zone of the drive sprocket of the transmission and to provide ashape compatible with the desired disposition of the coupling points ofthe four-bar linkage elements. The two links 18 and 19 are shortcomponents machined from aluminium, aluminium alloy, steel or othermaterial or made by forging, casting, stamping, fabricating, moulding orany other suitable method from metallic or other appropriate material ormaterials, preferably a material of light weight and high strength. Eachlink has two accurately machined bores providing locations fordeep-groove, full complement bearings, i.e. bearings with bearing ballsin direct contact with one another, rather than separated by webs of acage, and seated in deep grooves providing both radial and lateral(thrust) location of the balls. The bearings receive pivot pins fixed inthe swing arm and in the frame tubes or brackets attached thereto. Thetwo links can be identical in construction to provide savings inmanufacturing and purchasing cost.

The four-bar linkage constructed and arranged as described in thepreceding paragraph represents a particularly sturdy wheel-carryingstructure with good resistance to lateral flexing. This resistance ispromoted by the short lengths of the links 18 and 19 and theirdisposition at a spacing in the fore and aft direction of the bicycle;this spacing ensures that there is no coincidence, in a vertical ornearly vertical plane, of pivot points of the links such as to form ahinge zone reducing the transverse stiffness of the suspension.

The bicycle additionally includes a spring and damper unit 20 which ispivotably connected with the swing arm 17 in the vicinity of the bendtherein and with a bracket between the crossbar 11 and down tube 12. Theconnection of the unit 20 with the arm 17 can be adjustable to enablevariation in the orientation of the unit in a vertical plane relative tothe bicycle fore and aft direction.

As is evident from FIG. 1, the swing arm 17, under the control of theremaining elements of the four-bar linkage, is able to pivot upwardly inresponse to bumps encountered by the rear wheel 14 during use of thebicycle. In particular, the linkage is able to move between a first endsetting in an unloaded state of the bicycle, thus with the rear wheel ina lowermost position, and a second end setting in a fully loaded stateof the bicycle, thus with the rear wheel in an uppermost position. Theend settings are defined by, respectively, maximum extension and maximumcompression of the spring and damper unit 20. The lengths and relativedispositions and orientations of the links 18 and 19 are such thatduring travel of the linkage from the first end setting to the secondend setting the rearward link 18 pivots upwardly (at its rearward end)in the course of an initial phase of such travel while the forward link19 pivots upwardly (at its forward end) only very slightly. In thecourse of a further and final phase of such travel the rearward link 18continues to pivot upwardly while the forward link 19 now pivotsslightly downwardly. The forward link thus executes a reciprocatingmotion. This action of the links 18 and 19 produces a shift in theinstantaneous pivot centre of the linkage and consequently a desireddisplacement of the swing arm 17 and rear wheel 14 from the viewpoint ofcounteracting the bobbing effect induced in the chain-and-sprockettransmission by the power stroke exerted by the rider of the bicycle.The instantaneous pivot centre, which is denoted by 21 in FIG. 1, isdefined by the point of intersection of an axis 22 containing the pivotpoints of the rearward link 18 and an axis 23 containing the pivotpoints of the forward link 19.

The locus 24 of the instantaneous pivot centre 21 under upwardpivotation of the swing arm 17 is evident from the five diagrams ofFIGS. 2 a to 2 e, which illustrate different positions of the swing armin the course of such movement and thus different settings of thefour-bar linkage between its first setting (FIG. 2 a) and second endsetting (FIG. 2 e). As shown, the locus 24 describes—progressingrearwardly from the foremost disposition of the pivot centre 21—ashallow convex arc and then a shallow concave arc. The locus passesthrough the point of pivot connection of the lower link 19 with theframe, at which point the transition from the convex portion to theconcave portion of the locus occurs. The relatively small deviation ofthe locus from the axis 23 of the forward link 19 reflects the minimalpivotation of that link, which manifests itself during the initial phaseof movement of the linkage from the first to the second end setting as arelatively rapid rearward and downward travel of the instantaneous pivotcentre 21. Only in the final phase of travel of this pivot centre do thetwo links 18 and 19 move at similar rates through a similar angle, whichultimately produces a relatively slow movement of the centre and bringsit into close proximity with the axis 15 of the pedal crank drive. Asexplained in the introduction, this course of movement of the pivotcentre represents the desired course from the viewpoint of minimisingchanges in chain length so as to counteract bobbing or pedal feedback.In addition, the minimal change in chain length assists the action of achain-displacing or derailleur mechanism if provided.

The described pivot action of the four-bar linkage has an equallyadvantageous influence on the springing and damping supplied by thespring and damper unit 20. When the linkage is in its first end setting,i.e. the suspension unloaded (FIG. 2 a), the distance between the rearwheel axis 16 and the instantaneous pivot centre 21 is large and thewheel to spring/damper ratio is similarly large. In the second endsetting of the linkage (FIG. 2 e), the distance is significantly smallerand the wheel to spring/damper ratio is smaller. If the unit 20 has anappropriately adjustable point of pivot connection with the swing arm 17or frame 10, the inclination of the unit relative to the axis 23 of theforward link 19 can be altered. This effectively changes theperpendicular distance of the line of action of the unit 20 from thepivot centre 21 so as to enable possible selection of a rising, linearor falling wheel to spring/damper ratio; the particular configurationillustrated in the drawings provides a slightly rising rate.

A bicycle embodying the invention, thus a bicycle incorporating a rearwheel suspension as described in the foregoing, combines a sturdywheel-carrying structure with an intelligent wheel path under suspensionaction and additionally offers the possibility of an intelligent wheelto spring/damper ratio.

1. A bicycle comprising a frame, a rear wheel and a rear suspensionwhich comprises a swing arm carrying the rear wheel and two pivot linksspaced from one another in the fore and aft direction of the bicycle andcoupling the arm to the frame to form therewith a four-bar linkagemovable between a first end setting in an unloaded state of thesuspension and a second end setting in a loaded state of the suspension,wherein the links are each pivotably connected with the arm and theframe outwardly of the wheel circumference and are arranged so that onmovement of the linkage from the first to the second end setting aninstantaneous pivot centre defined by the intersection of two axes eachcontaining the pivot points of a respective one of the links movesdownwardly and rearwardly with respect to said direction at a first rateover an initial part of the travel of the pivot centre and at a secondrate reduced in relation to the first rate over a subsequent part of thetravel of the pivot centre.
 2. A bicycle as claimed in claim 1, whereina rearward one of the links pivots through a greater angle than aforward one of the links during an initial phase of movement of thelinkage from the first to the second end setting and throughsubstantially the same angle as the forward link during a final phase ofthat movement.
 3. A bicycle as claimed in claim 1 or claim 2, whereinthe locus of the instantaneous pivot centre during said movement thereofdescribes a first arc which substantially convex upwardly and a secondarc which is substantially concave upwardly.
 4. A bicycle as claimed inclaim 1, wherein the locus of the instantaneous pivot centre during saidmovement thereof passes through the pivot point of a forward one of thelinks at the connection thereof with the frame.
 5. A bicycle as claimedin claim 1, wherein the direction of said movement of the instantaneouspivot centre is generally towards the axis of a pedal crank drive of thebicycle.
 6. A bicycle as claimed in claim 5, wherein the instantaneouspivot centre is disposed adjacent to the axis of the drive in the secondend setting of the linkage.
 7. A bicycle as claimed in claim 1, whereina rearward one of the links extends forwardly from the arm with respectto the fore and aft direction.
 8. A bicycle as claimed in claim 1,wherein a forward one of the links extends rearwardly from the arm withrespect to the fore and aft direction.
 9. A bicycle as claimed in claim8, wherein the forward one of the links additionally extends downwardlyfrom the arm.
 10. A bicycle as claimed in claim 1, wherein a rearwardone of the links extends forwardly and a forward one of the linksrearwardly from the arm and the links extend convergently in directionaway from the rear wheel in the first end setting of the linkage.
 11. Abicycle as claimed in claim 1, wherein a rearward one of the links isconnected with the frame at a seat tube thereof.
 12. A bicycle asclaimed in claim 1, wherein a forward one of the links is connected withthe frame at a down tube thereof.
 13. A bicycle as claimed in claim 1,wherein the links are substantially identical.
 14. A bicycle as claimedin claim 1, wherein the links are connected with the swing arm and theframe by connecting means comprising deep-groove, full complementbearings.
 15. A bicycle as claimed in claim 1, wherein the links aremachined or moulded components.
 16. A bicycle as claimed in claim 1,wherein the links are connected with the swing arm and the frame byconnecting means comprising a single continuous pivot pin at each pivotpoint.
 17. A bicycle as claimed in claim 1, wherein a spring and damperunit is pivotably connected with the arm and frame to provide sprung anddamped travel of the linkage between its end settings.
 18. A bicycle asclaimed in claim 17, wherein the spring and damper unit is adjustable inposition relative to the arm and frame to vary the springing and dampingrates of the unit.